[OpenBIOS] Strict aliasing

Segher Boessenkool segher at kernel.crashing.org
Wed Aug 12 15:47:12 CEST 2009

> The reason OpenBIOS violates the strict aliasing rules so often is
> because it uses char arrays to represent endian-specific data.

No.  The reason it isn't valid C code is simply "because it isn't
valid C code".  Specifically, it is accessing arrays of chars as
another type.  This is not allowed.

> Char arrays may have alignment different from that of the longer  
> types.

That is not a problem on most architectures.  GCC allows it in most  

> That's why casting a pointer to such array to a pointer to a longer  
> type
> and dereferencing it is considered unsafe by the compiler.

It isn't considered "unsafe".  It is simply _not valid code_.  It is
impossible for a compiler to detect (or diagnose!) all invalid code;
but the compiler is allowed to assume it is presented with valid code,
it can do <whatever> with invalid code.

>   The compiler
> assumes that different types would not occupy the same memory  
> unless we
> tell it not to make such assumption.

It's not an assumption, it is guaranteed (by definition) that in any
valid C program, any object is only accessed as its stored type or a
character type.

> Char arrays serve as protection against misusing endian-specific  
> data as
> host-endian integers.  The alternative it to use sparse annotated  
> types,
> such as __le32.  __le32 has the same alignment as u32, but sparse  
> would
> recognize attempts to use __le32 in arithmetic operations.

A much better way is to access all data that needs marshalling via some
functions that do just that, e.g., read32be(const u8 *) and write16le 
(u8 *, u8) .
It is much more readable, and the compiler knows what you're doing,
because it is correct C code.


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